This invention relates to optical waveguides, and more particularly to optical waveguides that can be used in heat assisted magnetic recording.
Magnetic recording heads have utility in magnetic disc drive storage systems. Most magnetic recording heads used in such systems today are xe2x80x9clongitudinalxe2x80x9d magnetic recording heads. Longitudinal magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities.
Superparamagnetic instabilities become an issue as the grain volume is reduced in order to control media noise for high areal density recording. The superparamagnetic effect is most evident when the grain volume V is sufficiently small that the inequality KuV/kBT greater than 70 can no longer be maintained. Ku is the material""s magnetic crystalline anisotropy energy density, kB is Boltzmann""s constant, and T is absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the stored bits. Therefore, as the grain size is decreased in order to increase the areal density, a threshold is reached for a given material Ku and temperature T such that stable data storage is no longer feasible.
The thermal stability can be improved by employing a recording medium formed of a material with a very high Ku. However, with the available materials the recording heads are not able to provide a sufficient or high enough magnetic writing field to write on such a medium. Accordingly, it has been proposed to overcome the recording head field limitations by employing thermal energy to heat a local area on the recording medium before or at about the time of applying the magnetic write field to the medium. Heat assisted magnetic recording generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that the applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media. By heating the medium, the Ku or the coercivity is reduced such that the magnetic write field is sufficient to write to the medium. Once the medium cools to ambient temperature, the medium has a sufficiently high value of coercivity to assure thermal stability of the recorded information.
It is believed that reducing or changing the bit cell aspect ratio will extend the bit density limit. However, different approaches will likely be necessary to overcome the limitations of longitudinal magnetic recording.
An alternative to longitudinal recording that overcomes at least some of the problems associated with the superparamagnetic effect is xe2x80x9cperpendicularxe2x80x9d magnetic recording. Perpendicular magnetic recording is believed to have the capability of extending recording densities well beyond the limits of longitudinal magnetic recording. Perpendicular magnetic recording heads for use with a perpendicular magnetic storage medium may include a pair of magnetically coupled poles, including a main write pole having a relatively small bottom surface area and a flux return pole having a larger bottom surface area. A coil having a plurality of turns is located adjacent to the main write pole for inducing a magnetic field between the pole and a soft underlayer of the storage media. The soft underlayer is located below the hard magnetic recording layer of the storage media and enhances the amplitude of the field produced by the main pole. This, in turn, allows the use of storage media with higher coercive force, consequently, more stable bits can be stored in the media. In the recording process, an electrical current in the coil energizes the main pole, which produces a magnetic field. The image of this field is produced in the soft underlayer to enhance the field strength produced in the magnetic media. The flux density that diverges from the tip into the soft underlayer returns through the return flux pole. The return pole is located sufficiently far apart from the main write pole such that the material of the return pole does not affect the magnetic flux of the main write pole, which is directed vertically into the hard layer and the soft underlayer of the storage media.
When applying a heat or light source to the medium, it is desirable to confine the heat or light to the track where writing is taking place and to generate the write field in close proximity to where the medium is heated to accomplish high areal density recording. In addition, for heat assisted magnetic recording (HAMR) one of the technological hurdles to overcome is to provide an efficient technique for delivering large amounts of light power to the recording medium confined to spots of, for example, 50 nm or less. A variety of transducer designs have been proposed and some have been experimentally tested. Among these are metal coated glass fibers and hollow pyramidal structures with metal walls. For all these approaches, confinement of the light depends on an aperture which is to be fabricated into the end of the structure and gives this kind of transducer the name xe2x80x9caperture probes.xe2x80x9d Generally these devices suffer from very low light transmission rendering the devices useless for HAMR recording. For example, tapered and metallized optical fibers have demonstrated light confinement down to approximately 50 nm with a throughput efficiency of 10xe2x88x926. Pyramidal probes made from anisotropic etching of Si wafers have been designed with throughput efficiencies of 10xe2x88x924 for similar spot sizes. Although this is the state of the art, it is still about two orders of magnitude too small for HAMR.
Improvements in throughput efficiency have been achieved for these transducers by changing the taper angles, filling the hollow structures with high index materials, and by trying to launch surface plasmons (SP) on integrated edges and corners of these tip-like structures. Although doing so does increase the throughput to some extent, the most promising SP approach is still very inefficient due to a lack of an efficient SP launching technique. In addition, all aperture probes suffer from a lower limit on spot size which is twice the skin depth of the metal film used to form the aperture. Even for aluminum, the metal with the smallest skin depth for visible light, this corresponds to a spot size of xcx9c20 nm.
Solid immersion lenses (SILs) and solid immersion mirrors (SIMs) have also been proposed for concentrating far field optical energy into small spots. The optical intensity is very high at the focus but the spot size is still determined by the diffraction limit which in turn depends on the refractive index of the material from which the SIL or SIM is made. The smallest spot size which can be achieved with all currently known transparent materials is xcx9c60 nm, which is too large for HAMR.
A metallic pin can be used as a transducer to concentrate optical energy into arbitrarily small areal dimensions. The metallic pin supports a surface plasmon mode which propagates along the pin, and the width of the external electric field generated by the surface plasmon mode is proportional to the diameter of the pin. Smaller pin diameters result in smaller spots, and in principle the spot size can be made arbitrarily small. However, smaller pin diameters also result in much shorter propagation lengths for energy transport. In fact, for a 50 nm spot size the 1/e propagation length of the surface plasmon is typically substantially less than a micron. Therefore, a metallic pin by itself is not useful as a near field transducer.
There is a need for transducers that can provide a reduced spot size and increased throughput efficiencies.
This invention provides an apparatus for producing a small spot of optical energy comprising a planar waveguide shaped to direct a linearly polarized electromagnetic wave to a focal point within the waveguide, and a metallic pin positioned at the focal point whereby the electromagnetic wave creates surface plasmons on a surface of the pin.
The waveguide can include edges that are shaped to reflect the electromagnetic wave. The edges can have a substantially parabolic shape. The end of the waveguide can be truncated. The planar waveguide can alternatively include a mode index lens for directing the electromagnetic wave to the focal point. The metallic pin can be embedded in the waveguide and can extend through an end of the waveguide.
The apparatus can further comprise means for coupling a linearly polarized electromagnetic wave into the planar waveguide, and the means for coupling a linearly polarized electromagnetic wave into the planar waveguide can comprise a diffraction grating.
The apparatus can further comprise means for phase shifting a portion of the linearly polarized electromagnetic wave. The means for phase shifting can comprise a layer of material having a first section with a first refractive index and a second section with a second refractive index.
The planar waveguide can include a core layer and the means for phase shifting can comprise a first portion of the core layer having a thickness different from a second portion of the core layer.
The means for phase shifting can alternatively comprise a first diffraction grating and a second diffraction grating, where the first diffraction grating and the second diffraction grating are offset in a longitudinal direction.
The apparatus can further include a cladding layer adjacent to one or both sides of the planar waveguide. The cladding layer can have different thicknesses to provide means for phase shifting the electromagnetic wave.
In another aspect the invention can include a means for coupling a linearly polarized electromagnetic wave into the planar waveguide to excite a transverse magnetic waveguide mode. Furthermore, the waveguide thickness can be chosen to be slightly greater than the waveguide cutoff thickness for the transverse magnetic mode.
In another aspect, the invention encompasses an apparatus comprising a pyramidal structure having four substantially flat sides converging to a point, a first material lying adjacent to an exterior of each of the sides and having a first index of refraction, a second material lying adjacent to an interior of each of the sides and having a second index of refraction, and a metallic pin positioned adjacent to the point. The apparatus can further include means for phase shifting a portion of an electromagnetic wave prior to reflection by the sides.
The invention further encompasses an apparatus comprising a conical structure having a first refractive index, a layer of material on a surface of the conical structure, the material having a second refractive index lower than the first refractive index, a metal layer positioned on the layer of material, and a metallic pin positioned adjacent to a point of the conical structure.
The invention also encompasses a recording head comprising a magnetic write pole, a planar waveguide positioned adjacent to the magnetic write pole, the planar waveguide shaped to direct a linearly polarized electromagnetic wave to a focal point within the waveguide, and a metallic pin positioned at the focal point whereby the electromagnetic wave creates surface plasmons on a surface of the pin.
The invention further encompasses a recording head comprising a magnetic write pole, and a waveguide positioned adjacent to the magnetic write pole, the waveguide including a pyramidal structure having four substantially flat sides converging to a point, a first material lying adjacent to an exterior of each of the sides and having a first index of refraction, a second material lying adjacent to an interior of each of the sides and having a second index of refraction, and a metallic pin positioned adjacent to the point.
Another aspect of the invention encompasses a recording head comprising a magnetic write pole, and a waveguide positioned adjacent to the magnetic write pole, the waveguide including a conical structure having a first refractive index, a layer of material on a surface of the conical structure, the material having a second refractive index, lower than the first refractive index, a metal layer positioned on the layer of material, and a metallic pin positioned adjacent to a point of the conical structure.
The invention further encompasses a disc drive comprising means for rotating a storage medium, and means for positioning a recording head adjacent to a surface of the storage medium, wherein the recording head comprises a magnetic write pole, a planar waveguide positioned adjacent to the magnetic write pole, the planar waveguide being shaped to direct a linearly polarized electromagnetic wave to a focal point within the waveguide, and a metallic pin positioned at the focal point whereby the electromagnetic wave creates surface plasmons on a surface of the pin.